Here we describe the cloning of the chicken homologue of Barx-1, its expression during embryonic development and the regulation of its expression by FGF-8 and BMPs in developing maxillary primordia. ![]() Barx-1 is one such gene that is expressed in discrete regions of the mouse facial primordia and therefore, may control facial patterning (Tissier-Seta et al., 1995). Furthermore, BMPs and FGFs can differentially control various processes in the developing embryo, including the regulation of the expression of Pax-9, a paired-box gene, in the presumptive dental mesenchyme (Neubüser et al., 1997 Buckland et al., 1998 Ericson et al., 1998).Īs homeobox-containing genes are candidates for patterning the facial primordia, it is important to understand signaling interactions that control their differential expression. For example, ectopic application of BMP-4 can induce Msx-1 gene expression, resulting in bifurcation of skeletal elements, while FGFs can partially substitute for the facial epithelium in its ability to maintain outgrowth of the primordia (Barlow and Francis-West, 1997 Richman et al., 1997). Of particular interest are Bmp-4 and Fgf-8, which are expressed in the epithelium and have been proposed to control outgrowth of the facial primordia (Francis-West et al., 1994 Barlow and Francis-West, 1997 Richman et al., 1997). These include members of the bone morphogenetic protein family (BMP) and fibroblast growth factor family (FGF). Therefore, knowledge of which epithelial signals control the expression of homeobox-containing genes may give us insight into mechanisms that pattern the facial primordia.Ī number of signaling factors have been implicated to control outgrowth and patterning of the facial primordia (reviewed in Francis-West et al., 1998). Some homeobox genes are essential for facial development, and it has been proposed that their differential expression controls patterning of the facial structures (Qiu et al., 1995 1997 reviewed by Francis-West et al., 1998). In addition, these interactions have been shown to control the expression of a few homeobox-containing genes such as Msx-1 (Takahashi et al., 1991). Epithelial–mesenchymal signaling interactions control outgrowth of the primordia (Wedden, 1987). The vertebrate face develops from the coordinated growth of the facial primordia, which initially consist of undifferentiated mesenchymal buds covered by an epithelial layer. This provides evidence that the differential expression of FGF-8 and BMPs may determine homeobox-containing gene expression and hence patterning of the facial primordia. This suggests that in vivo, FGF-8/BMP signaling may regulate Barx-1 gene expression. By contrast, BMPs reduce Barx-1 expression and can antagonize FGF-8 signaling. We show that epithelial signals are required to maintain Barx-1 expression and that FGF-8 can substitute for the epithelium. In the maxillary primordia, mesenchymal expression of Barx-1 is complementary to that of Msx-1, which correlate with overlying epithelial expression of Fgf-8 and Bmp-4, respectively. Here we describe the isolation of the chick homologue of the homeobox-containing gene, Barx-1, and show its expression in the developing facial primordia, stomach, and appendicular skeleton. The vertebrate face develops from a series of primordia surrounding the primitive mouth and is thought to be patterned by the differential expression of homeobox-containing genes.
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